1. Field of the Invention
This invention relates to an optical recording/reproducing apparatus, and more particularly to an optical pickup apparatus, that is adaptable for various types of optical recording media having different layout conditions such as recording density. Also, the present invention is directed to an optical recording/reproducing apparatus using the optical pickup apparatus.
2. Description of the Related Art
Generally, an optical recording/reproducing apparatus for driving disc-type media, such as a compact disc(CD) (which are well known as recording media making use of a laser light beam), records or reproduces data by irradiating a laser beam onto the recording face of a disc while rotating the disc. To this end, the optical recording/reproducing apparatus includes an optical pickup for irradiating a laser beam generated from a light source, such as a semiconductor laser, onto the recording face of the optical disc using optical system devices such as an objective lens.
Recently, a digital versatile disc (DVD) is now commercially available that is capable of storing a larger amount of information than the conventional CD. The DVD is usually designed for use with a light source having a different number of aperture and a different wavelength from the CD. In this case, the wavelength and the number of aperture of a light beam is related to the size of beam spot. The size of beam spot is selected from the standpoint of minimizing an effect caused by cross-talk between signal tracks on the recording face of the optical disc. Accordingly, since the DVD (with a larger recording density than the CD) has a small track pitch, the size of beam spot must also be smaller than that of the CD. In this case, a scheme making use of a shortened wavelength and an increased aperture number can be considered for reducing the size of beam spot. This approach may be viable since the size of a beam spot is directly proportional to the wavelength of the light beam while being inversely proportional to the number of aperture as seen from the following formula:                     d        =                  k          ⁢                      λ            NA                                              (        1        )                            wherein d represents the size of beam spot, k is a constant, λ is a wavelength of a light beam, and NA is the number of aperture of an objective lens. It can be seen from the formula (1) that a short wavelength and a large aperture number is used as to obtain a smaller size of beam spot when using a DVD as compared to the CD. For instance, an optical pickup for accessing the CD uses a light beam with a wavelength(λ) of 780 nm and an objective lens with the number of aperture of 0.45, whereas an optical pickup for accessing the DVD uses a light beam with a wavelength(λ) of 650 nm and an objective lens with the number of aperture of 0.6. Also, in the DVD, a light beam is sensitive to the thickness of the disc as the number of aperture of a light beam changes. The depth of the recording face, that is, the depth of the light transmission layer, is set to have a smaller value in a DVD than that of the CD. In other words, a noise component increases so that data cannot be recorded or reproduced due to an increase in the optical aberration when a light is transmitted by means of an objective lens with an aperture number of 0.6 through a light transmission layer with a thickness equal to that of a CD. Thus, the thickness of the light transmission layer in the DVD is set to have a smaller value than that in the CD. For instance, a light transmission layer of the CD has a thickness of 1.2 mm while a light transmission layer of the DVD has half the thickness thereof, that is, a thickness of 0.6 mm.        
An optical pickup for changeably accessing both a CD and DVD must include two light sources generating a different wavelengths of light beams and two objective lenses with a different number of aperture. Where an optical pickup is provided with two light sources and two objective lenses, problems occur in that the optical pickup size becomes large, its structure is complicated, and manufacturing costs increases. In order to solve these problems, an optical pickup has been used where it has a single light source and means for appropriately controlling the number of aperture of the objective lens depending on the corresponding disc to thereby access the CD and the DVD.
For example, Japanese Patent Laid-open Gazette No. Pyung 9-185839 has disclosed an optical pickup that can access two types of optical discs of different thicknesses with light transmission layers by controlling the number of aperture of an objective lens employing a liquid crystal filter and a polarizing filter. The optical pickup controls the number of aperture of the objective lens into two modes by turning the liquid crystal filter on or off depending on whether or not a voltage is applied, to thereby selectively change the polarization characteristic of a light beam generated from a light source, and by allowing the polarizing filter to selectively shut out a portion of the light beam in accordance with the polarization characteristic of a light beam changed by means of the liquid filter.
Also, Japanese Patent Laid-open Gazette No. Pyung 9-198704 has disclosed an optical pickup that is capable of accessing two types of optical discs by providing two objective lenses with a single lens supporting member in the twin-lens system to thereby switch a position of the objective lens in accordance with a rotation of the lens supporting member.
A different approach involves blue lasers. Blue lasers generate a significantly lower wavelength of light beam as compared with conventional red laser beams. The blue laser is a light source that is expected to be commercially available in accordance with development of GaN system laser. It is reported that a wavelength band of the blue laser is approximately 400 nm. A next-generation optical disc employing such a blue laser, hereinafter referred to as “HD-DVD”, requires a light source with a different wavelength along with an objective lens with a corresponding number of aperture. Particularly, the HD-DVD requires a larger number of aperture because short wavelength of a blue light beam allows it to have a higher density such that its beam spot size must be smaller than the spot used with the DVD. More specifically, a wavelength (λ) of a light beam and the number of aperture NA applied to each of the HD-DVD, the DVD and the CD have a relationship in the following formula:λ1<λ2<λ3NA1≧NA2≧NA3  (2)
wherein λ1, λ2 and λ3 are wavelengths of light beams corresponding sequentially to the blue laser disc, the DVD and the CD, and NA1, NA2 and NA3 are the number of aperture corresponding sequentially to the blue laser disc, the DVD and the CD. Accordingly, the size of beam spots irradiated onto the three types of optical discs has a relationship in the following formula (3) when the above formulas (1) and (2) are applied.                                           d            ⁢                                                   ⁢            1                    <                      d            ⁢                                                   ⁢            2                    <                      d            ⁢                                                   ⁢            3                          ⁢                                  ⁢                                            λ              ⁢              1                                      NA              ⁢                                                           ⁢              1                                <                      λ                          NA              ⁢                                                           ⁢              2                                <                                    λ              ⁢              3                                      NA              ⁢                                                           ⁢              3                                                          (        3        )                            wherein d1, d2 and d3 represent the size of beam spots irradiated onto the HD-DVD, the DVD and the CD, respectively. As described above, the HD-DVD, the DVD and the CD have a different beam spot size due to a difference in the layout condition such as a recording density, etc. Accordingly, when it is intended to interchangeably access the three types of optical discs with a single optical recording/reproducing apparatus, three light sources and three objective lenses are required in the conventional method. However, when the optical pickup includes three light sources and three objective lenses, it has problems in that its structure becomes complicated and that the manufacturing costs increase. Accordingly, it is necessary to provide an optical pickup apparatus that is capable of accessing at least three types of optical discs with a different layout condition as well as having a minimum of constituent elements.        